I am mortified to be creating a Star Wars writeup, but I actually had a question about the movies that had not already been answered in everything2, so I thought I would share the results of my research with all.
In Return of the Jedi, as rebel commanders introduce the stolen plans for the reconstructed Death Star, Mon Mothma says
Many Bothans died to bring us this information.
The actress delivers this line with a scenery-chewing excess of emotion, and got laughs when I watched the 'Jedi' re-release a few years ago. It left me with the question, what is a Bothan?
Though the creatures never appear in the Star Wars movies, they apparently got plenty of coverage in the various comic books, CD-ROMs, and other products connected with the series. Trolling through a couple of the roughly infinite number of Star Wars fan sites on the Internet, I can report the following:
Bothans stand 1.5 meters tall, are covered in fur, and omnivorous. As a race, they have spent centuries developing a massive spy network. During the 'Galactic Civil War' (the events depicted in Star Wars movies, episodes 3-6), the Bothans sold information to both sides though they favored the rebels. It is unclear how this preference manifested itself; perhaps Princess Leia was allowed to buy Imperial secrets at a discount.
The tale of the capture of the second Death Star plans is told in detail in the book Shadows of the Empire. Two dozen Bothan pilots and gunners of their 'Blue Squad' died raiding an Imperial transport called Suprosa. Luke Skywalker participated in the raid as well.
Bothans come from a planet called Bothawui, though they also have numerous colony worlds in the surrounding space.
They live in family clans, and the websites I cite below can give you all sorts of details about decoding a Bothan's clan membership from his name, as well as some subtleties of Bothan politics.
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Sources:
Star Wars: Behind The Magic CD-Rom , as quoted in
http://www.thetrukstop.com/starwars/sote/bothan.html
http://www.slicerhq.com/~nabubu/citizens/races/bothans.html